EXAMPLE 7.1 Estimating the Average Time Spent on a Smartphone
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CASE 7.1 The following data are the daily number of minutes for an SRS of 8 students at your institution:
| 117 | 156 | 89 | 72 | 116 | 125 | 101 | 100 |
We want to find a 95% confidence interval for , the average number of minutes per day a student uses his or her smartphone.
The sample mean is
and the standard deviation is
with degrees of freedom . The standard error of is
From Table D, we find . The margin of error is
The 95% confidence interval is
| 1.895 | 2.365 | |
| 90% | 95% | |
Thus, we are 95% confident that the average amount of time per day a student at your institution spends on his or her smartphone is between 88.3 and 130.7 minutes.